An X-ray CT apparatus is one kind of radiotomography apparatus. An X-ray CT apparatus irradiates X rays while rotating a rotating part equipped with an X-ray tube and an X-ray detector around the circumference of an object to be examined, and detects X rays that are transmitted through the body of the object with the X-ray detector.
With this kind of X-ray CT apparatus, cardiography is performed by arranging the cardiac X-ray transmission data for a desired cardiac phase according to the following method.
Data is collected so that the cardiac phase and the angle of the rotation direction (hereunder, referred to as “view angle”) are not the same for each rotation.
Desired data is acquired from the above described collected “view angle” data by interpolation.
In this case, the term “desired cardiac phase” refers to, for example, immediately before a contraction phase in which the cardiac motion is comparatively stable. Since a tomogram can be constructed by collecting X-ray transmission data from at least a 180° direction around the object, the data of a desired cardiac phase is collected from a 180° direction or more while rotating the rotating part a plurality of times.
When performing the above described kind of cardiography with an X-ray CT apparatus, if a large number of projection images can be acquired at one time the X-ray transmission data can be obtained in a shorter scanning time. Collection of data in a short scanning time in this manner relates to the control of radiation exposure such as X rays, and is desirable for the object. Methods for acquiring a large number of projected images at one time include not only a view method, but also a method that uses X rays in the shape of a cone beam that spreads out in the direction of the body axis of the object. A plurality of X-ray detecting elements are disposed not only in the view direction that is the rotational direction, but also in the body axis direction that is the direction of the circulation axis, in correspondence to the plane of projection of the cone-beam X rays.
Patent Document 1 discloses technology that breaks down X-ray transmission data that includes a cardiac phase of interest into data segments using this type of cone-beam X-ray CT apparatus, and then combines a plurality of these data segments to reconstruct a cardiac tomogram at a certain section.
The term “data segment” refers to consecutive data that was obtained continuously in a view direction that includes X-ray transmission data for a certain cardiac phase, and the length in the circulation axis direction thereof and the length in the view direction depend on the scanning speed, the heart rate, the table feeding speed and the like.
In the case of cardiography performed by a helical scan using cone-beam X rays as disclosed in Patent Document 1, the number of data segments used to reconstruct a tomogram is fixed as the minimum number among the number of segments corresponding to each view angle area.
Patent Document 1: Japanese Patent Application Laid-Open No. 2003-93378